The present invention relates to a sintered body of a titanium-based carbonitride having improved properties particularly when used as an insert material in curing tools for the machining of metals by, for example, turning, milling and drilling, especially under heavy wear conditions.
Titanium-based carbonitride alloys, so-called cermets, are today well established as an insert material in the metal cutting industry and are especially used for finishing. They contain mainly carbonitride hard constituents embedded in a binder phase. The hard constituent grains generally have a complex structure with a core surrounded by a rim of another composition. Their grain size is usually &lt;2 .mu.m.
In addition to Ti, other metals of the groups IVa, Va and VIa, i.e., Zr, Hf, V, Nb, Ta, Cr, Mo, and/or W, are normally found in the carbonitride hard constituents but may also be present as carbide and/or nitride hard constituents. The binder phase generally contains cobalt as well as nickel. The amount of binder phase is generally 3-25% by weight.
U.S. Pat. No. 4,447,263 discloses inserts of a titanium-based carbonitride alloy provided with a wear resistant surface layer of carbonitride or oxycarbonitride alone or in combination. The layer is obtained by a heat treatment at 1100.degree.-1350.degree. C. in an atmosphere of N.sub.2, CO and/or CO.sub.2 at subpressure.
Inserts according to the above-mentioned patent, thus, consist of a brittle layer on a brittle substrate resulting in an inadequate tool life under toughness demanding operations. It has now surprisingly been found that, if the heat treatment is performed at atmospheric pressure, preferably overpressure, an enrichment of binder phase under the above-mentioned surface layer is obtained, which gives improved toughness.